e into the closest
competition, the advantages of diversification of structure, with the
accompanying differences of habit and constitution, determine that the
inhabitants, which thus jostle each other most closely, shall, as a
general rule, belong to what we call different genera and orders.

The same principle is seen in the naturalisation of plants through man's
agency in foreign lands. It might have been expected that the plants
which would succeed in becoming naturalised in any land would generally
have been closely allied to the indigenes; for these are commonly looked
at as specially created and adapted for their own country. It might
also, perhaps, have been expected that naturalised plants would have
belonged to a few groups more especially adapted to certain stations in
their new homes. But the case is very different; and Alph. de Candolle
has well remarked, in his great and admirable work, that floras gain by
naturalisation, proportionally with the number of the native genera and
species, far more in new genera than in new species. To give a single
instance: in the last edition of Dr. Asa Gray's "Manual of the Flora of
the Northern United States," 260 naturalised plants are enumerated, and
these belong to 162 genera. We thus see that these naturalised plants
are of a highly diversified nature. They differ, moreover, to a large
extent, from the indigenes, for out of the 162 naturalised genera,
no less than 100 genera are not there indigenous, and thus a large
proportional addition is made to the genera now living in the United
States.

By considering the nature of the plants or animals which have in any
country struggled successfully with the indigenes, and have there become
naturalised, we may gain some crude idea in what manner some of the
natives would have had to be modified in order to gain an advantage over
their compatriots; and we may at least infer that diversification of
structure, amounting to new generic differences, would be profitable to
them.

The advantage of diversification of structure in the inhabitants of the
same region is, in fact, the same as that of the physiological division
of labour in the organs of the same individual body--a subject so well
elucidated by Milne Edwards. No physiologist doubts that a stomach by
being adapted to digest vegetable matter alone, or flesh alone, draws
most nutriment from these substances. So in the general economy of
any land, the more widely and perfe